b'Magnetics in the mountainsFeatureforward modelled response from a uniform earth and the upward continued pseudomag transformed topography. Clearly the idea had merit as the pseudomag transformed topography image is very similar to the forwarded modelled response from a homogeneous earth with topography.If we re-scale the upward continued, pseudomag transformed topography to the same range as the observed data and re-plot Figure 3 (see Figure 5) we see an excellent match, further supporting the potential of this as a fast approximation of the topographic response.The relationship between the size of the topographic anomaly and its magnetic response is not always intuitive, and quite small hills or shoulders can result in significant magnetic anomalies.As it appears that the pseudomag transform has been deprecated or is missing in at least one of the common commercial processing packages, it is worth pausing for a moment to describe it. The equation for the pseudomag transform is derived and given in Blakely (1995) (p347, Eq 12.47 and 12.48) and in full space form, with all the definitions on the same page in Li & Oldenburg (1998) (their Eq 1 and 2). For those who prefer to skip the technical part I have included an extract of the relevant parts from my source code in Appendix 1. There is also an older school F77 version in the excellent uSGS Potential Field Software package contained in uSGS Open-File Report 97-725. In their 1998 paper Li and Oldenburg describe a logical process to derive the conversion constant between gravity data and magnetic data. Knowing the conversion constant is only relevant in transforming the pseudomag units back to gravity units, as you would do after inverting a pseudomag gravity dataset to recover density from the pseudo-susceptibility. Prior to the Li and Oldenburg paper being published, I also thought that the magnetic inversion code might be used to invert gravity data, and so I wrote the pseudomag code. The choice of parameters for the conversion constant, while different to Li and Oldenburg, to my reckoning, followed an equally logical process. However, as the constant is applied to both the real and imaginary parts of the Fourier transform, it just becomes a constant offset in the space domain. When the transform is applied to topographic data we are talking about transforming Figure 4.Images from the study area of Figure 2 showing; top left,meters to nanoTeslas. The physical relationship between the two topography from SRTM30, top right , observed magnetic field, bottom left,escapes me, so until someone can link the two the value of the calculated magnetic field using MAGFOR3D, bottom right, upward continued,constant becomes academic.pseudomag transformed topography. All images use a linear colour scale.from successive planes, it would fail once the plane hit theLi and Oldenburg also discuss problems with noise generated by highest point in the survey as the observation plane had to bethe pseudomag transform, which as you can see from the code above the ground. It seemed that the only available empiricalsnip combines a rotation with a power term which behaves in a approach was with the finite element codes, which were goingsimilar way to a second vertical derivative. This noise is partially to take time. We could send the problem out to a bureau to rundealt with by the upward continuation to flying height, but can on a series of clusters, but not many clients are prepared to paybe almost completely removed with a Lanczos or Hanning filter for this type of modelling, particularly at bureau rates. while doing the FFT. Ive included the Lanczos and Hanning code in Appendix 1 for anyone starting from scratch.One day, when looking at the sections in Figure 1 and othersMoving back to the large survey area in Indonesia. Having like them, it occurred to me that if I thought of the topographicestablished that the pseudomag transform produced a surface as a gravity profile I could generate the modelledsimilar result to the forward model, I generated the transform magnetic field by rotating the topography to an equivalentfor use in the interpretation. It took less than a minute to run magnetic field using the reverse of Baranovs pseudo-gravitythe process on a 18034 x 7284 SRTM grid covering a much transformthe pseudo-magnetic transform. larger area than needed for the survey. As a fall back and Returning to the data set from Figure 2, I applied a pseudomagfinal check, I also generated a forward model that, because transform to the topography and upward continued it by 60of the size of the area, had to be broken into 5 blocks. The m to match the magnetic data and model. Figure 4 showstotal mesh size was 5624 x 1701 x 144 with observation a comparison of the topography, observed magnetic data,points at the centre of each voxel column. This was started JuNE 2020 PREVIEW 36'